You dirty rat

Rats are formidable, athletic colo­nisers. You may have heard of Razza, the notorious Norway rat deliberate­ly let loose on a rat-free island near Stewart Island as part of a study of rat movements led by Rachel Fewster. The rat had been released by PhD student James Russell, who then ended up los­ing a season of field-work when the radio-tagged creature promptly disap­peared, only found several months later on a nearby island, having swum 400 m to get there. Razza was written about in Nature, newspapers all over the world and was sympathetically portrayed in Witi Ihimaera’s The Amazing Adven­tures of Razza the Rat.

Meanwhile Fewster and her colleagues continued their research, more recently turning their attentions to Pearl Island, also off Stewart Island, which had three species of rats (Norway, ship and kiore) until they were simultaneously eliminated in August 2005. Then, nine months later, both Norway and ship rats reappeared.

DOC didn’t know whether the rats had reinvaded, or whether they hadn’t been successfully eradicated in the first place. The latter would have been bad news indeed, suggesting DOC’s eradication programmes weren’t working as well as they should. However, through genetic analysis of a few stray rat tails left over from previous studies, Fewster and co could confirm the rats came from Stewart Island, probably on their own mettle.

Fewster’s research on rat DNA adds ballast to growing consensus that the best way to keep rat-free islands that way is to ensure the surrounding islands are also rat-free, that conservation efforts should be aimed at clusters of islands rather than individuals. This approach is exemplified by the Motu Kaikoura Trust, which plans
to eliminate rats on 26 islands along the west coast of Great Barrier in the Hauraki Gulf. By the time of printing—assuming all went to plan—the project would have finished its aerial drop of rat bait on all of them.